The present invention relates to a bolt capable of measuring a mechanical quantity.
As a technology that monitors strain of a bolt, method, in which a slot is provided in an axial direction (lengthwise) of a bolt and optical fibers are inserted into the slot to measure strain, and method of embedding a strain gauge into the slot, have been put into practical use. Since a fiber cable and a lead wire must be taken outside from a head of a bolt in these technologies to become an obstacle at the time of bolt clamping, however, the technologies are not suited to applications, in which strain in many bolts is monitored constantly. Hereupon, it is thought of, as a solution, to embed a strain sensor, such as strain gauge, etc., in a bolt and to take out measured values through display or radio waves. JP-A-11-118637 is listed as a patent therefor, and shows a sensor bolt comprising a sensor, an amplifier for sensed values, and a battery built in the bolt and a display provided in the bolt for display in the case where a measured strain becomes equal to or less than a set value.
Strain sensors such as strain gauges, quartz sensors, etc. have a very small output value relative to strain while being large in temperature dependency. Therefore, a serious problem comes out in accuracy of measured values in the case where temperature distribution is involved in a bolt as frequently experienced in clamp bolts of a piping system. Thus, it is an object of the present invention to provide a bolt with a function of measuring strain, having a favorable accuracy of measurement even in the case where temperature distribution is involved in the bolt, and eliminating connection of a lead wire outside the bolt.
Also, with conventional examples as in the above-described patent, etc., a battery, a strain sensor, an amplifier, etc. are built in so as to prevent a lead wire from being exposed outside a bolt, in view of a problem in workability. However, there is caused a problem that a sensor is limited in service life because of being battery-operated. Since ordinary strain sensors consume a battery due to flow of electric current of a current value in the order of mA, the number of measurements of an amount of strain is limited. Since bolts on bridge piers, in tunnels, or the like cannot be in many cases exchanged easily, it is essential to extend a service life in order to increase the applications. Thus, it is another object of the present invention to provide a bolt with a function of measuring strain, eliminating connection of a lead wire outside the bolt and enabling use without caring for the service life of a battery.